Various vehicles or machines such as wheel loaders use a torque converter in their powertrains between their engine and transmission. The torque converter can be used in a locked or an unlocked mode when the machine is moving, which provides for a desirable torque control feature for some machines that may encounter relatively immovable obstacles. For example, a wheel loader may push a bucket against a pile of material, a ripper may encounter a boulder, and so forth. Torque converter slippage is relied upon to provide a transmission output torque control benefit, for example, when contact with the obstacle is initiated, because it results in limiting transmission input torque and reducing the possibility of low engine lug issues, such as engine underspeed or stalling. However, torque converter slippage and other losses have been found to increase overall fuel consumption of the machine when compared to other power transmission methods such as continuously variable transmissions (CVTs).
CVTs provide a continuously variable torque capability, which is an improvement over a traditional toque converter/transmission powertrains. A typical CVT employs a torque controlling element that provides a continuously variable torque or speed transmission capability. One known application of CVTs for machine use is embodied as a split torque transmission, which includes a drive train that is powered by dual inputs—one input being a torque- or speed-controlled input, such as from a hydraulic variator, and the other being a direct power input from an engine. These two inputs are combined in a planetary gear arrangement, which includes outputs driving the various gear ratios of the transmission.
In split torque transmissions, it is desirable to accurately control the variator such that the resultant system operation quickly tracks operator inputs. For example, a response lag time to an operator input may cause a temporary discrepancy between the expected and actual operation of the system, which may result in deteriorated shifting performance thus causing operator discomfort, system inefficiency, and/or increased drive train wear. In some instances, such time lag may cause engine underspeed and/or stall. In other words, as efficient as CVTs are in terms of fuel economy, they are also known to be relatively slow to respond to operator commands in that time delays between an operator command and a change in the operation of the variator, and thus the transmission, are known to exist. These time delays are especially evident when a machine having a CVT-assisted powertrain such as a wheel loader encounters an obstacle. In such conditions, the delay between an operator command to stop moving against the obstacle, and an actual change in motion of the machine, may cause engine underspeed or even stalling.
Various solutions have been proposed in the past to improve the response of CVT transmissions. One such example can be found in U.S. Patent Application Pub. No. 2011/0087411 A1 (“Fuller”), which was published on Apr. 14, 2011. Fuller describes an electronic controller for controlling a mechanical variator device. In the system of Fuller, CVT response is improved by differentiating a control pressure request with respect to time, for example, by numerically differentiating the pressure request and multiplying the differential by a factor, to obtain a compensation value. The compensation value is applied to modify a control pressure request sent to a hydraulic valve arrangement that controls the reaction torque of the variator, and essentially operates as a derivative-type term of the command signal in the control scheme of the transmission. Although the compensation value acquired in this fashion may be effective in improving the control stability of the transmission by counteracting inherent system dampening, it would be ineffective in specifically addressing engine underspeed issues when an immovable object is encountered because it depends on the rate of change of the operator command, which can vary depending on the driving style of a particular operator and thus provide unpredictable operating response of the system under all conditions.